Human beings are the ultimate Frugivores and Seed Dispersers in June 2010

Anthropochory or Human-Mediated Dispersal (HMD)

Since the genus Homo originated in Africa, diversified and dispersed into Asia, and throughout the other continents, he contributed to dispersal of many plants across the earth. This process is called anthropochory (Hodkinson & Thompson 1997; Vittoz & Engler 2007.) First, travelling by walking, second, by floating and later driving, finally by flying, various species of fruit-eating Homo has been able to transport seeds and plants from one continent to another. Behaving as non-human frugivores and seed dispersers which colonized new continent or empty newly-formed volcanic islands (see review), Humans thus certainly affected plant diversity in both directions, by eating fruit and dispersing plants on the one hand, by enriching diversity on the other hand, that of the botanical garden of Montpellier, for instance. Homo also contributes to the spread of invasive species with ultimate consequences for the endemic flora, and for fauna. This topic known as Human-Mediated Dispersal (HMD) is now intensively analysed and debated in the literature, showing how human activities impact the occurence and distribution of plant species through altering land, but also directly by dispersal of propagules (Nathan et al. 2008, Wichmann et al. 2009)

Seed dispersal by walking

Finally, even whether one may ask you to remove your shoes at Airport security, Human beings still travel with shoes, and seeds. Wichmann et al. (2009) have demonstrated that many seeds are carried by humans on shoes, half of them falling off within 5m, seeds being regularly still attached to shoes after 5km. Mechanistic modelling have shown that the primary vector of dispersal, wind, was less important as an agent of long-distance dispersal, dispersing seeds less than 250 m. The results of this study suggest that walking humans can disperse seeds to very long distances, up to at least 10 km, and provide some of the first quantified dispersal kernels for HMD. When considering, that human beings also travel by boat, car and plane, then dispersal kernel for HMD might reach several thousands of km.

Seed dispersal by touring

Whatever their mode of travel, similarly to deer that carry seeds on fur, Human beings also transport seeds on their clothes. Recently, Pickering and Mount (2010) assessed the potential for tourists to act as unintentional seed dispersal agents. They reviewed published and unpublished data on seed dispersal via clothing (picture), vehicles (cars) and in/on horses and donkeys, all of which can be used by tourists. A total of 754 species - a majority of them from vehicules, i.e. 505 spp) - of seeds from terrestrial plants have been collected from these vectors, 15% of which are internationally recognised environmental weeds. Most were herbs (429 species) or graminoids (237 species) and native to Europe. Tourists as thus acting as human-mediated dispersal agents and increase the risk of spreading insasive species.

Seed dispersal by driving

Agricultural equipment can also disperse weed seeds over large distances. Boyd and White (2009) examined the potential impact of harvesting equipment on weed seed dispersal within and between blueberry fields. (picture) Seed loads were examined on harvesting equipment between fields and results suggest that harvesting equipment is a major vector of seed dispersal. Secondary dispersal within blueberry fields by harvesting equipment is inevitable. Dispersal may be reduced by avoiding dense weed patches, or altering harvest timing. Periodic cleaning of harvesting equipment between fields will help prevent the spread of weed seeds.

Seed dispersal by flying

Aerial transportation of human beings is another efficient, modern mean of long-distance seed dispersal throughout the planet. Shimono & Konuma (2008) emphasized that international trade is a major route by which non-indigenous organisms are introduced into new habitats. They studied the factors that affect the initial assemblage of plant species introduced by the international grain trade. By surveying weed seed contamination of spring wheat imported from Canada to Japan, they found that many other crop plants contaminated the wheat. Because various transportation vehicles, temporary storage sites and port elevators are used commonly with all exported crops and it is difficult to remove all residues from them, other crops might be carried over into the wheat commodity.

Impact of dispersal of Homo sapiens on frugivores and seed dispersal

One of the major consequences of the recent dispersal of Human beings throughout the continents is the massive extinction of megafauna in all continents. Key examples are of the loss of island endemics include the dodo (Raphus cucullatus) from Mauritius, moas (e.g., Dinornis maximus) from New Zealand, and elephant birds (Aepyornis maximus) from Madagascar. Megafaunal collapse during the late Pleistocene can largely be traced to a variety of negative human impacts, such as overharvesting, biological invasions, and habitat transformation (Sodhi et al. 2009). To survive, many plants with Megafaunal fruits had to be consumed and dispersed by surrogates, among them Human beings as emphasized by Guimarães et al. (2009) and Guix (2009). The extinction of Megafauna due to Human beings spread had several potential consequences, such as a scale shift reducing the seed dispersal distances, increasingly clumped spatial patterns, reduced geographic ranges and limited genetic variation and increased among-population structuring. These effects could be extended to other plant species dispersed by large vertebrates in present-day, defaunated communities.

On the other hand, White et al. (2004) investigated the effects of distance from a seed source on dispersal of seeds to three restored rainforest sites (all approximately 10 years old) on the Athertou Tablelands, North Queensland. They concluded that recovery of distant sites to self-sustaining mature rainforest may not occur without Human-Mediated Dispersal. Mammal-dispersed species in particular are unlikely to reach these sites unless contributed by humans. Recovery of tropical rainforest systems is likely to occur more quickly and be more successful if restoration is concentrated around the perimeter of existing rainforest rather than in isolated patches.